Apparatus and method for granulating plastic strands

ABSTRACT

A novel apparatus is described for granulating plastic strands (4) which emerge from nozzles (3) in molten form and are supplied to a following, downward inclined runoff channel (1) in which the plastic strands are transported by a liquid stream, in particular water stream. Further the apparatus has a dewatering zone (1b) for the plastic strands (4) and a granulator (6) disposed after this dewatering zone. Between the dewatering zone (1b) and the granulator (6) there is at least one tempering zone (9) comprising a rigid, unmoving channel (15) and integrated transport rolls (16) for the plastic strands (4) and having a length such that the temperature differences of the plastic strands (4) across their cross section are largely equalized during their holding time in the tempering zone (9). 
     After the plastic strands (4) have been transported through the tempering zone (9) they are introduced into the granulator (6) in which the strands are granulated by one or more knife assemblies and then discharged as corresponding plastic granules.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an apparatus and method for granulatingplastic strands that emerge from nozzles in molten form and are suppliedto a following, downward inclined runoff channel in which the plasticstrands are transported by a liquid stream, in particular water stream,the apparatus further having a dewatering zone for the plastic strandsand a granulator disposed after said dewatering zone for granulating thesolidified plastic strands with the aid of at least one knife assembly.

2. Description of the Related Art

U.S. Pat. No. 5,182,115 discloses an apparatus for cooling, drying andgranulating strands emerging from nozzles in molten form having a runoffchannel disposed with its receiving-side end below the nozzles, andfurther having a device for producing a coolant stream on the runoffchannel, while the runoff channel is followed by a granulator andfurthermore a dewatering zone is disposed in the runoff channel beforethe discharge-side end, the runoff channel being provided in saiddewatering zone with openings for free passage of the coolant.

In addition the runoff channel in this known apparatus is provided inits bottom in the area after the dewatering zone with admission nozzlesfor a gas or air stream over such a length and so close together thatthe strands are guided for removal of residual water along the runoffchannel in largely frictionless fashion with respect to the bottom intothe granulator disposed at its discharge-side end with a degree ofdryness permitting immediate processing. This granulator has essentiallytwo draw-in rolls for the plastic strands one above the other within acorresponding housing, further a knife roll cooperating with acorresponding counterknife. This knife roll and the counterknifegranulate the plastic strands supplied by the draw-in rolls, and thegranules thus produced by the granulator fall out through a dischargeshaft for further processing.

In addition U.S. Pat. No. 4,180,539 discloses an apparatus of theabove-defined type wherein the runoff channel is followed by an upwardinclined dewatering and drying zone for the plastic strands thatconsists essentially of a circulating, endless and latticed conveyerbelt on which the plastic strands leaving the runoff channel and therebyentraining at least part of the liquid stream are transported furthertoward the granulator. Above this circulating latticed conveyer belt anumber of air nozzles are disposed for directing relatively sharp airjets ("air knives") onto the surface of the plastic strands. These airknives serve to dry the strands on their surfaces during transportation,whereby they blow away the water sticking thereto and remove it downwardthrough the lattice belt. The latticed formation of the transport beltis necessary in the known apparatus of U.S. Pat. No. 4,180,539 inparticular because relatively sharp air jets are directed onto thestrands from above so that the air must likewise be removed downwardthrough the lattice belt since the strands would otherwise not have theright contact pressure on the surface of the lattice belt; the strandssubjected to the air knives would instead "fly up" and "swirl around" asit were. In addition, the known dewatering and drying zone ensures onlya dewatering and drying process with respect to the strand surfaceswhile the interior of the strands still has an elevated temperaturecompared to the surface, so that the plastic strands entering thegranulator directly after the dewatering and drying zone are not yetequalized in their temperature profile.

It is fundamentally necessary, however, that the plastic strands to beprocessed into granules be neither too soft nor too brittle when theyare supplied to the knife roll of the granulator. In view of thisrequirement and also because of the extremely different properties ofthe plastics marketed in granular form, these properties ranging from"very elastic" to "extremely brittle", the cooling devices used betweenan extruder and a granulator in known apparatuses are already adapted bythe manufacturer to the type of plastic later to be processed in orderto ensure that the newly extruded plastic strands are cooled with suchan intensity that they have the desired strength properties when theyreach the knife roll of the granulator. When processing certainplastics, in particular glass fiber-reinforced plastics, one requiresthat the granulating temperature be over about 100° C., whereby thetemperature profile should be as homogeneous as possible across thestrand cross section. The same requirement also holds quite generallyfor example in the processing of plastics that are fundamentallyrelatively poor heat conductors.

In the apparatus known from U.S. Pat. No. 4,180,539 the dewatering anddrying process does not permit the plastic strands to be given ahomogeneous temperature distribution across the entire strand crosssection before they enter the granulator, because the dewatering anddrying process concentrates in particular on the strand surfaces.

SUMMARY OF THE INVENTION

In view of the above-described facts and disadvantages in knownapparatuses for cooling and granulating plastic strands, the presentinvention is based on the problem of providing an improved apparatus ofthe above-defined type that is designed in such a way as to ensure thatthe strands entering the granulator have a temperature required fortheir further processing and in particular a virtually homogeneoustemperature profile across the strand cross section.

This problem is solved according to the invention in an apparatus of theabove-defined type by disposing at least one tempering zone between thedewatering zone and the granulator, said tempering zone comprising arigid, unmoving channel and integrated transport rolls for the strandsand having a length such that the temperature differences of the strandsacross their cross section are largely equalized during the holding timein the tempering zone.

This design makes it in particular possible to control the holding timeof the plastic strands within the tempering zone in accordance with thetransport speed adjusted for the transport rolls which form anintegrated part of the tempering zone or the corresponding channel.

The inventive design of the apparatus for granulating plastic strandsyields the essential advantage that this apparatus has a tempering zonewith its own transport function so that in particular upon a start thestrands are automatically "threaded" into the granulator.

The tempering zone provided according to the invention can be alignedessentially horizontally, but it can also be provided with an eitherdownward or upward inclination to the horizontal toward the granulator.

Finally the channel of the tempering zone can be formed as a vibratingchannel in order to strengthen and support the desired effect oftemperature equalization within the plastic strands.

The inventive design results in a self-transporting temperatureequalizing zone with a threading function within the apparatus forgranulating these plastic strands.

BRIEF DESCRIPTION OF THE DRAWINGS

The method according to the invention comprises the steps of: supplyingthe molten strands to a downwardly inclined runoff channel; cooling andtransporting the plastic strands within the runoff channel by a waterstream; dewatering the plastic strands in a dewatering zone of therunoff channel; equalizing temperature differences of the plasticstrands across a cross section thereof at a temperature over about 100°C. in a tempering zone following the dewatering zone; and granulatingthe plastic strands with a granulator disposed after the tempering zone.

The invention will be explained more closely in the following using anexample, reference being made to the enclosed drawing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The single FIGURE shows a schematic side view of an apparatus forcooling and granulating plastic strands in its operating position.

This apparatus has schematically shown nozzle assembly 2 which issupplied in a way not specified here but known in the art with moltenthermoplastic plastic which is then pressed out of nozzle 3 disposedbelow. Normally a plurality of such nozzles 3 are located in a row sideby side below nozzle assembly 2.

The FIGURE shows the operating position of the apparatus in whichplastic strands 4 emerging from nozzles 3 first pass onto the initialarea of inclined runoff channel 1 which is provided with cooling andtransport water feed pipe 8 at its upper end shown on the left in theFIGURE Runoff channel 1 divided functionally into a first channelportion, cooling zone 1a, and a second portion, dewatering zone 1b,guides supplied plastic strands 4 in parallel arrangement with the aidof the supplied cooling and transport water and by reason of itsinclined arrangement further toward tempering zone 9 directly followingdewatering zone 1b and disposed in the area between runoff channel 1 andgranulator 6 shown on the right.

Tempering zone 9 serving to equalize temperature differences withinplastic strands 4 consists essentially of rigid, unmoving channel 15 andtransport roll system 16 integrated into channel 15 and having lowerfeed roll 16" and upper counterpressure roll 16' for furthertransporting plastic strands 4 coming from runoff channel 1 towardgranulator 6 following tempering zone or temperature equalizing zone 9.

Transport roll system 16 is disposed essentially at the beginning ofchannel 15, whereby gap or nip 5 between lower feed roll 16" and uppercounterpressure roll 16' has a separation height such that strands 4entering gap 5 are given a feeding force toward granulator 6 by means offrictional engagement but without essential deformation. Gap or nip 5 ofpair of rolls 16', 16" is located approximately at the height of thebottom of channel 15.

In the shown example, the length of tempering zone 9 is approximatelyequal to the length of runoff channel 1, but tempering zone 9 canpreferably also be longer than runoff channel 1 provided that itsminimum length is such that the temperature differences of each of theplastic strands across their respective cross sections are largelyequalized.

Granulator 6 has housing 17 provided on its side facing tempering zone 9with admission opening 7. Admission opening 7 receives the associatedend area of channel 15 such that homogeneously solidified plasticstrands 4 transported further thereon are introduced into granulator 6with a self-threading effect, as it were.

Within housing 17 of granulator 6 there are two draw-in rolls forplastic strands 4, upper draw-in roll 10 and lower draw-in roll 11.Draw-in rolls 10 and 11 draw strand 4 now gradually leaving temperingzone 9 further into granulator 6 and guide it finally to rotatingshearing knife 13 cooperating with anvil knife 12 disposed on knifeholder 12a. This knife assembly granulates supplied plastic strands 4,the resulting plastic granules leaving housing 17 at outlet 14. In thearea of dewatering zone 1b a gravimetric preliminary dewatering takesplace, the water being collected by collecting box 18 below dewateringzone 1b.

Tempering zone 9 is aligned essentially horizontally in the shownexample, but it is also possible to provide tempering zone 9 with aneither downward or upward inclination to the horizontal towardgranulator 6, this being omitted in the drawing for simplicity's sake.

Further, it is possible to give at least channel 15 of tempering zone 9a vibrating design such that plastic strands 4 are set vibrating duringtheir further transport through tempering zone 9.

The design of the granulating apparatus according to the presentinvention permits plastic strands 4 transported from tempering zone 9toward granulator 6 to reach admission opening 7 of granulator housing17 with the desired homogeneous temperature or degree of solidificationacross the strand cross sections, virtually without regard for the typeof plastic primarily used. Plastic strands 4 now have a rigidity suchthat they can be threaded automatically into granulator 6 and thengranulated easily with the aid of the knife assembly therein. Thehomogeneous temperature profile of the strands yields high-qualitygranules with a perfect cut edge and low dust content over a longproduction time.

We claim:
 1. An apparatus for granulating plastic strands (4) whichemerge from nozzles (3) in molten form and are supplied to a following,downward inclined runoff channel (1) in which the plastic strands aretransported by a water stream, the apparatus further having a dewateringzone (1b) for the plastic strands (4) and a granulator disposed afterthe dewatering-zone (6) for granulating the solidified plastic strands(4),wherein between the dewatering zone (1b) and the granulator (6)there is provided at least one temperature equalizing zone (9)comprising a temperature equalizing channel (15) arranged to cool thestrands without adding external heat, the temperature equalizing zonehaving a minimum length such that the temperature differences of each ofthe plastic strands (4) across their respective cross section arelargely equalized during their holding time in the temperatureequalizing zone (9) in the absence of external heat to such an extentthat they have a homogeneous degree of solidification across their crosssection when entering into the granulator (6), and wherein a transportroll system (16) having a gap (5) is provided, said transport rollsystem engaging and further transporting the plastic strands (4) comingfrom the runoff channel (1) toward the granulator (6).
 2. The apparatusof claim 1 wherein the length of the temperature equalization zone (9)is at least equal to or greater than the length of the runoff channel(1).
 3. Apparatus according to claim 1, wherein the transport rollsystem (16) is disposed essentially at the beginning of the temperatureequalizing channel (15).
 4. Apparatus according to claim 1, wherein thetransport roll system (16) is integrated in the temperature equalizingchannel (15).
 5. Apparatus according to claim 1, wherein the transportroll system (16) comprises an opposing pair of rolls (16', 166"), thegap (5) of the pair of rolls (16', 16") having a height such that thestrands (4) are frictionally engaged and given a feeding force towardthe granulator (6) by the rolls without essential deformation, said gap(5) being located approximately at the height of a bottom of thetemperature equalizing channel (15).
 6. Apparatus according to claim 1,wherein the temperature equalizing channel (15) is aligned essentiallyhorizontally.
 7. Apparatus according to claim 1, wherein the temperatureequalizing channel (15) is disposed with an upward inclination to thehorizontal toward the granulator (6).
 8. Apparatus according to claim 1,wherein the temperature equalizing channel (15) is of vibrating design.9. Apparatus according to claim 1, wherein the length of the temperatureequalizing channel (15) is such that the equalized temperature of theplastic strands (4) across each strand's cross section is over 100° C.during each strand's holding time in the temperature equalization zone(9).
 10. Apparatus for granulating plastic strands (4) which emerge fromnozzles (3) in molten form and are supplied to a following, downwardinclined runoff channel (1) in which the plastic strands are transportedby a water stream, the apparatus further having a dewatering zone (1b)for the plastic strands (4) and a granulator disposed after thedewatering zone (1b) for granulating the solidified plastic strands(4),wherein between the dewatering zone (1b) and the granulator (6)there is provided at least one temperature equalizing zone (9)comprising a temperature equalizing channel (15) arranged to cool thestrands without adding external heat, the temperature equalizing zone(9) being at least equal to or greater than the length of the runoffchannel (1) and wherein a transport roll system (16) having a gap (5) isprovided said transport roll system engaging and further transportingthe plastic strands (4) coming from the runoff channel (1) toward thegranulator (6).
 11. Apparatus according to claim 10, wherein thetransport roll system (16) is disposed essentially at the beginning ofthe temperature equalizing channel (15).
 12. Apparatus according toclaim 10, wherein the transport roll system (16) is integrated in thetemperature equalizing channel (15).
 13. Apparatus according to claim10, wherein the transport roll system (16) comprises an opposing pair ofrolls (16', 16"), the gap (5) of the pair of rolls (16', 16") having aheight such that the strands (4) are engaged frictionally and given afeeding force toward the granulator (6) by the rolls without essentialdeformation, the gap (5) being located approximately at the height of abottom of the temperature equalizing channel (15).
 14. Apparatusaccording to claim 10, wherein the temperature equalizing channel (15)is aligned essentially horizontally.
 15. Apparatus according to claim10, wherein the temperature equalizing channel (15) is disposed with anupward inclination to the horizontal toward the granulator (6). 16.Apparatus according to claim 10, wherein the temperature equalizingchannel (15) is of vibrating design.
 17. Apparatus according to claim10, wherein the length of the temperature equalizing channel (15) issuch that the temperature of each plastic strands (4) across its crosssection is largely equalized at a temperature over 100° C. during itsholding time in the temperature equalization zone (9).
 18. A method forgranulating plastic strands which emerge from nozzles (3) in moltenform, comprising the steps of:supplying the molten strands (4) to adownwardly inclined runoff channel (1); cooling and transporting theplastic strands (4) within the runoff channel (1) by a water stream;dewatering the plastic strands (4) in a dewatering zone (1b) of therunoff channel (1); transporting the plastic strands (4) coming from therunoff channel (1) through a temperature equalization zone (9)comprising a temperature equalizing channel (15) toward a granulator (6)disposed after the temperature equalization zone (9) by means of atransport roll system (16) having a gap (5) for engaging and advancingthe plastic strands (4); equalizing temperature differences across thecross-section of the plastic strands (4) by moving the dewatered plasticstrands (4) along a minimum length of the temperature equalizing channel(15) without adding external heat sufficient to cause the plasticstrands to each reach an equalized temperature across their respectivecross-section to be in condition for granulation; and granulating theplastic strands (4) with the granulator (6) disposed after thetemperature equalization zone (9).
 19. The method for granulatingplastic strands (4) according to claim 18, further comprising the stepof:transporting the plastic strands (4) through the temperatureequalization zone (9) via integrated transport rolls (16', 16")comprising the transport roll system (16).
 20. The method forgranulating plastic strands (4) according to claim 18, furthercomprising the step of:selecting a height of the gap (5) of thetransport roll system (16) such that the plastic strands (4) are engagedand given a feeding force toward the granulator (6) by frictionalengagement but without essential deformation of the strands.
 21. Themethod for granulating plastic strands (4) according to claim 18,further comprising the step of:aligning the temperature equalizationzone (9) such that the plastic strands (4) pass therethrough essentiallyhorizontally.
 22. The method for granulating plastic strands (4)according to claim 18, further comprising the step of:aligning thetempering zone (9) such that the plastic strands (4) pass therethroughat an upward inclination toward the granulator (6).
 23. The method forgranulating plastic strands (4) according to claim 18 further comprisingthe step of:vibrating the plastic strands in the tempering zone (9). 24.The method for granulating plastic strands (4) according to claim 18such that the temperature differences of each plastic strand (4) acrosseach strand's cross-section is largely equalized at a temperature over100° C. during each strand's holding time in the temperatureequalization zone (9).